To debug my latest PHPLib commits I needed
some output before session_start() without sending
any headers to the browser. Well, I thought of using 
error_log() that should not output anything but write 
to my logfile. I changed my php.ini to: 

display_errors = Off    
log_errors     = On    
error_log      = [mypath]/logs/

Now I hoped that I'd be allowed to use error_log() before 
session_start(), here's a simple example:

<?php
error_log("I don't like that!");

session_start();
session_register("counter");

print ++$counter;

error_log("counter = $counter");
error_log(" ")
?>

But PHP bailed at me in the error_log:

I don't like that!
PHP Warning:  Cannot send session cookie - headers already sent in [mypath]/bug.php3 on line 4
[mypath]/bug.php3(4) : Warning - Cannot s
end session cookie - headers already sent
PHP Warning:  Cannot send session cache limiter - headers already sent in
[mypath]/bug.php3 on line 4
[mypath]/bug.php3(4) : Warning - Cannot s
end session cache limiter - headers already sent
counter = 1 

Hmm, did I forget a auto_prepend file, could I be doing any output before session_start()? Let's check the results of a command line telnet request.

HTTP/1.1 200 OK
Date: Thu, 31 May 2001 13:38:27 GMT
Server: Apache/1.3.14 (Unix) PHP/4.0.5-dev mod_ssl/2.7.1 OpenSSL/0.9.5a
X-Powered-By: PHP/4.0.5
Connection: close
Content-Type: text/html

1Connection closed by foreign host.

No, I did not output anything before session_start(). Then I uncommented the first error_log() call and everything was just fine. The error log showed:

counter = 1

counter = 2

... 

The problem is the same with 4.0.4pl1 (as one could expect).